Abstract:

The localization of mRNAs is a mechanism that is widely spread in most of the eukaryotic cells to spatially define protein synthesis and thus contributes to maintenance and generation of cellular asymmetry, neuronal function and embryonic development. In budding yeast localized mRNAs are forming RNA-protein (RNP) complexes with the mRNA-binding protein She2p, the adaptor protein She3p, that is also known to bind to mRNAs and the motor protein She1p (Myo4p). The mRNA binds to She2p by its cis-acting elements (zipcodes) and She2p is then associated to Myo4p via She3p that acts as an adapter. The Myo4p-She3p co-complex that is an important part of the core locasome and thus essential for the mRNA localization machinery is also involved in the inheritance of cortical ER (cER; Estrada P. et al., 2003). The mRNP is transported towards the barbed ends of actin cables consisting of actin molecules and tropomyosin filaments located at the bud tip. The process by which the mRNAs are anchored at the bud tip is unknown (Jansen R.P. et al., 1996; Gonsalvez G.B. et al., 2005). This She1-3 machinery is known to localize about 24 mRNAs to the bud tip including 16 mRNAs that encode secreted or membrane proteins like the aforementioned IST2 mRNA or the She2p transport-dependent EAR1 mRNA (Giaever G. et al., 2002; Terashima H. et al., 2002; Leon S. et al., 2008). RNP particles form the initial element for the segregation of cER. The fact that RNPs can localize together with tubular ER structures was also supported by the ASH1 mRNA which binds to the tip of ER-membrane tubules (Schmid M. et al., 2006) suggesting a coordination of mRNA localization and cER distribution.
This study demonstrates that mutants that are defective on ER segregation and morphology are at the same time affecting mRNA localization of early expressed mRNAs. This indicates a direct link between mRNA localization and ER inheritance. It was shown that the RNA-binding protein She2p associates independently of polysomes with ER membranes. This and the mRNP localization experiments with WSC2 mRNA demonstrate that the mRNA localization occurs independent on its translation.
Based on this study it was shown that the proper segregation of cER is essential for the localization of a subset of mRNAs that are transported early during cell growth and then get expressed at the time of tubular ER movement into the bud. Thus, a new model of early and late mRNA localization pathways linked to ER inheritance during the early stages of budding was established. mRNAs that are expressed early during the cell cycle are therefore co-localized with the ER, while late expressed mRNAs are localized towards the bud ER-independently. This temporally split mRNA localization represents the most time and energetically efficient transport mechanism during budding. Altogether, this study confirmed the hypothesis of a link between mRNA localization and ER transport.
Additionally, this study confirms that the tetramerization of She2p, the basic helical hairpin motif and the helix E (Niessing D. et al., 2004; Müller M. et al., 2011) are essential for the proper co-localization of tubular ER and RNPs that are depending on She2p. The mRNA-binding protein She2p can bind to artificial and protein-free liposomes depending on their curvature with a preference to a diameter of 30 nm comparable to the diameter of ER tubules in yeast. Because of the fact that the binding of She2p to membranes is saturable, She2p has an unexpected membrane binding property and thus is a likely candidate for the association of specific mRNAs to ER tubules.
She2p is therefore not only a protein that binds mRNA in the nucleus and thus initiates the mRNA transport mechanism but is also able to bind to liposomes and therefore can also be characterized as a lipid-binding protein that recognizes membrane curvature. So She2p is an ideal coordinator of mRNA transport and ER tubules.